This invention relates generally to lithography and, more particularly, to a photomask for projection lithography at or below about 160 nm and a lithography method using that mask.
Optical lithography has been one of the principal driving forces behind the continual improvements in the size and performance of the integrated circuit (IC). Feature resolution down to 0.18 xcexcm is now possible using the 248 nm Hg lamp or KrF excimer laser wavelengths and optical projection tools operating at numerical apertures above 0.55. The industry is now at a point though where resolution is limited for current optical lithographic technologies. In order to extend capabilities for the next millennium toward sub-0.13 xcexcm, modifications in source wavelength, optics, illumination, masking, and process technology are required and are getting a tremendous amount of attention.
With respect to masking, control of the phase information at a mask may allow for manipulation of imaging performance to achieve smaller feature resolution. Phase shift masking (PSM) employs constructive and destructive interference to improve both resolution and focal depth for a variety of feature types. For dense features, a transparent phase shifter added to or subtracted from alternating mask openings allows for a doubling in resolution by decreasing the mask and wafer electric field frequency. A lens acting on this diffracted mask information has a 50% decrease in the numerical aperture requirements. Phase shift masking using such an alternating shifter approach can also result in reduced sensitivity to defocus and other aberrations, but is limited to dense grating type mask features. Variations in the alternating phase shift mask have been developed to allow for application to non-repetitive structures. Depending on the mask fabrication technique, process control may limit the manufacturability of these types of phase shift masks for short UV wavelength exposures. Each of these prior phase-shift masking approaches also requires some level of added mask and process complexity and none of these techniques can be used universally for all feature sizes and shapes. Accordingly, an approach which can minimize mask design and fabrication complexity may gain the greatest acceptance for application to manufacturing.
An attenuated phase shift mask (APSM) may be such an approach. Using this approach, conventional opaque areas on a binary mask are replaced with partially transmitting regions (up to 100%) that produce a xcfx80 phase shift with respect to clear regions as disclosed in U.S. Pat. No. 4,890,309 to Smith et al. which is herein incorporated by reference.
In an ASPM, as light travels through a transparent material, a phase shift occurs based on the refractive index ni(l) and thickness t of the medium. By design, APSM films are not transparent but possess a transmissive characteristic based on the material""s extinction coefficient k(l). As k(l) increases, an abrupt phase shift also occurs at the air-film interface. As these materials are considered, it is the complex refractive index n*=n+ik that is of interest. The phase shifting effect as light travels through a semitransparent material can be expressed as:
xcfx86=(2xcfx80/xcex)[ni(xcex)xe2x88x921]t+arg [2n*2/(n*1+n*2)]
where n*1 and n*2 are complex refractive indices of the first and second medium respectively. This equation assumes that the phase contribution from the APSM film-substrate interface is negligible, which is a reasonable approximation. A third term to the equation above could be included to account for this phase change if desired. The transmission properties of an APSM film are determined as:
T=exp(xe2x88x924xcfx80kt/xcex).
Preferably, the APSM should provide transmission above about 0.5 percent for wavelengths at or below about 160 nm and also provide about a 180xc2x0 phase shift.
Additionally, an APSM which comprises a single layer film (or a material that can be processed as a single layer film) is most attractive from the standpoint of mask process complexity. There are several requirements for a single layer APSM material in order for it to be considered production worthy. Materials must exhibit suitable optical transmission and phase shifting properties and allow for pattern delineation (etching), radiation stability, and durability. Additionally, there may be optical requirements of the material at longer wavelengths, since any mask must be compatible with inspection and alignment operations.
The APSM film should also possess adequate etch characteristics and selectivity to both the resist and the substrate. Conventional wet etching is not anisotropic and limitations are being realized for current mask applications. Plasma-reactive ion etch (RIE) will be a requirement for these next generation masking materials, which presents both chemical and physical challenges to pattern transfer. Without sufficient selectivity to the mask substrate, etching of the mask substrate will contribute to phase shifting effects and thus will need to be accounted for. As exposure wavelengths are pushed below 160 nm, etch control becomes increasingly critical. Without adequate etch selectivity, a transparent etch stop layer would be required between the APSM film and the substrate. Suitable materials for use as an etch stop are increasingly difficult to identify at short UV wavelengths.
Work in areas of attenuated phase-shift masking has demonstrated both resolution and focal depth improvement for a variety of feature types. However, prior to the present invention practical materials for use in an APSM for IC mask fabrication which can satisfy the 180xc2x0 phase-shift and the required transmittance, at targeted wavelengths below about 160 nm, such as 157 nm, 148 nm or 126 nm, within a given film thickness, and which satisfy the other requirements noted above have not been found.
Several materials, such as CrO, CrN, CrF, AIN, SixNy, TASiO, MoO, MoN, ZrN, ZrO, or other variations or combinations of these materials on a fused silica mask substrate, have been explored for use as masking films for use at the longer wavelengths, such as 365, 248 and 193) nm. Unfortunately, these materials are generally not the best starting point for the design of an APSM for sub-160 nm applications since absorption of both APSM materials and fused silica substrates increase significantly below 180 nm. Also, some combinations of these materials lead to poor etch selectivity between the mask and the mask substrate.
A mask for use in a lithography process in accordance with one embodiment of the present invention includes a masking film made of at least one material with at least a silicon component which provides a transmission above about 0.5 percent and a phase shift of about a 180 for radiation at a wavelength at or below about 160 nm.
An attenuated phase shift mask in accordance with another embodiment of the present invention includes a substrate with at least one surface and a masking film made of at least one material with at least a silicon component. The masking film is located on at least a portion of the one surface of the substrate and provides a transmission above about 0.5 percent and a phase shift of about a 180xc2x0 for radiation at a wavelength at or below about 160 nm.
A method for lithography in accordance with another embodiment of the present invention includes a few steps. First, a masking film made of at least one material with at least a silicon component is placed over at least a portion of the one surface of a substrate. Next, the masking film and the substrate are exposed to radiation at a wavelength at or below about 160 nm. The masking film provides a transmission above about 0.5 percent and a phase shift of about a 180xc2x0 for radiation at a wavelength at or below about 160 nm.
Another attenuated phase shift mask for use in a lithography process in accordance with another embodiment of the present invention includes a mask comprising at least silicon which provides a transmission below about 0.5 percent for radiation at a wavelength at or below about 160 nm.
Another mask in accordance with another embodiment of the present invention includes a substrate with at least one surface and a masking film on at least a portion of the one surface of the substrate. The masking film is made of at least silicon and provides a transmission below about 0.5 percent for radiation at a wavelength at or below about 160 nm.
Another method for lithography in accordance with another embodiment of the present invention also includes a few steps. First, a masking film made of at least silicon is placed over at least a portion of one surface of a substrate. Next, the masking film is exposed to radiation at a wavelength at or below about 160 nm. The masking film provides a transmission below about 0.5 percent for radiation at a wavelength at or below about 160 nm.
The present invention provides a number of advantages including providing a mask which has desirable optical properties for use in optical lithography at or below about 160 nm. More specifically, the masking film is made of at least one material with a silicon component and is capable of producing a phase shift of 180xc2x0 with transmission at a targeted wavelength at or below about 160 nm (such as 157 nm, 148 nm, or 126 nm) above about 0.5 percent and up to 100 percent. The material or materials used in the masking film (that is the thin film which controls the phase shifting and transmission of sub-160 nm radiation) are based on the unique extinction coefficient properties of silicon and/or silicon dielectrics (including silicon dioxide, SiO2, and silicon nitride, Si3N4) below 160 nm. Preferably, the substrate is essentially transparent. The low extinction coefficient value of silicon below 160 nm allows it to be incorporated at a significant level in the masking film to permit tailoring of the optical properties of the mask which is not possible at longer lithographic wavelengths. Additionally, the higher extinction coefficient of silicon for wavelengths above 160 nm (from a value near 1.6 to a value near 3.0 at 300 nm), allows for the control of longer wavelength transmission in the mask to be sufficiently low to allow for mask inspection and mask alignment.
Another advantage of the present invention is that the desired short and long wavelength transmission properties of the mask as well as its thermomechanical and exposing radiation stability properties can be customized through the use of additional materials in the masking film. More specifically, absorbing metal oxides, such as oxides of Ta, Mo, Ti, Fe, Ru, Mn, Cu, Cr, Ni, V, Nb, Hf, Sn, In, and Co, can be combined with silicon dioxide to decrease transmission properties of the APSM film below 160 nm and at longer wavelengths. Similarly, absorbing metal nitrides, such as nitrides of Ta, Mo, Ti, Fe, Ru, Mn, Cu, Cr, Ni, V, Nb, Hf, Sn, In, and Co, can also be combined with silicon nitride to decrease transmission properties of the APSM film below 160 nm and at longer wavelengths.
The present invention also provides suitable selectivity, preferably about 10:1 or better, between the mask and the underlying substrate as the result of the combination of a material with a silicon component in the mask and material with a fluoride component in the substrate. Materials with silicon are volatile in fluorine chemistry, whereas materials with fluorine are stable in the same chemistry. As a result, with the present invention the mask can be etched without a significant loss of the underlying substrate or resist material. Alternatively, a fused silica substrate (SiO2) can be used without a fluoride film when selectivity is less than 10:1 with the masking film, but etch control is achieved through careful process control, i.e. etch through the masking film 12 then stop at the surface of the substrate 14.
Yet another advantage of the present invention is that reflectivity at the masking film/air interface may be reduced through use of an optical interference coating, such as a silicon-based dielectric film (such as silicon nitride or silicon dioxide) with thickness adjusted to result in absorption and quarter wave interference at the targeted wavelength at or below 160 nm.
Yet another advantage of the present invention is that the mask can be modified for use in applications where the transmission is below about 0.5% and phase shifting is not required. In this particular embodiment, the mask is a binary mask and the masking film thickness is adjusted to control transmission properties at the targeted wavelength below 160 nm.